Regarding the "rattle". I had a Magnum 52 that did the same thing. Adding a head gasket helped but did not solve the problem. Running it on 5% nitro pretty much made it go away. Maybe OS has decided to build their engines to run on lower cost fuels. I buy 5% fuel for about $15 per gallon. (Europe uses mostly 0 nitro fuel.)
I have my doubts OS is building their engines for lower Nitro fuels - especially those being imported to the United States.